Implement Conics in SkCurveMeasure

src/utils/SkCurveMeasure.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCurveMeasure_DEFINED
 #define SkCurveMeasure_DEFINED
 
 #include "SkPathMeasurePriv.h"
 #include "SkPoint.h"
 #include "SkNx.h"
 
 // These are weights and abscissae for gaussian quadrature with weight function
 // w(x) = 1
 static SkScalar weights8[8] = {0.3626837833783620f, 0.3626837833783620f,
                                0.3137066458778873f, 0.3137066458778873f,
                                0.2223810344533745f, 0.2223810344533745f,
                                0.1012285362903763f, 0.1012285362903763f};
 static SkScalar absc8[8] = {-0.1834346424956498f, 0.1834346424956498f,
                             -0.5255324099163290f, 0.5255324099163290f,
                             -0.7966664774136267f, 0.7966664774136267f,
                             -0.9602898564975363f, 0.9602898564975363f};
 
 static Sk8f weights = Sk8f::Load(weights8);

[caryclark, 2016/08/15 19:13:04]

Writable globals are seldom a good idea. Another thread may run before this is
initialized. Put all of this where it is used, not here. It is not part of the
interface; it is part of the implementation.

[Harry Stern, 2016/08/15 20:55:16]

These should all have be static const - they never get written to. Should I
still put them as class members of ArcLengthIntegrator or are they fine as long
as they're const?

 static Sk8f absc = 0.5f*(Sk8f::Load(absc8) + 1.0f);
 
 
 class ArcLengthIntegrator {
 public:
     ArcLengthIntegrator() {}

[caryclark, 2016/08/15 19:13:04]

Why does this leave fSegType uninitialized?
Do you ever call this constructor? If not, remove it.

[Harry Stern, 2016/08/15 20:55:16]

I think I need the default constructor for the CurveMeasure constructor:
SkCurveMeasure contains an ArcLengthIntegrator and it doesn't get initialized
until the bottom of the SkCurveMeasure constructor. I don't know a better way to
do this.

     ArcLengthIntegrator(const SkPoint* pts, SkSegType segType);
     SkScalar computeLength(SkScalar t);
 
 private:
     SkSegType fSegType;
 
     // precomputed coefficients for derivatives in Horner form
     Sk8f xCoeff[3];

[caryclark, 2016/08/15 19:13:04]

Preceed with 'f'

[Harry Stern, 2016/08/15 20:55:17]

Done.

     Sk8f yCoeff[3];
+
+    SkScalar fConicW = 0;

[caryclark, 2016/08/15 19:13:04]

Why zero? That's a valid weight; do you mean for it to mean something special?
If you want to detect uninitialized fields, choose a number that is negative or
NaN.

Also, if this is a special value, document it.

[Harry Stern, 2016/08/15 20:55:17]

This variable only gets read when we are a conic, and it will always be
initialized to a correct value in such a case. Should I just make it
uninitialized or set it to an invalid value e.g. NaN as you suggest?

 };
 
 class SkCurveMeasure {
 public:
     SkCurveMeasure() {}
 
     // Almost exactly the same as in SkPath::Iter:
     // kLine_SegType  -> 2 points: start end
     // kQuad_SegType  -> 3 points: start control end
     // kCubic_SegType -> 4 points: start control1 control2 end
     // kConic_SegType -> 4 points: start control end (w, w)

[caryclark, 2016/08/15 19:13:04]

Do you repeat w twice? Do you rely on this? Why?

[Harry Stern, 2016/08/15 20:55:17]

I do set it to (w, w) for no particular reason, but I don't rely on it. I'm
pretty sure everywhere I use it, I just take the x value.

     //
     // i.e. the only difference is that the conic's last point is a point
     // consisting of the w value twice
     SkCurveMeasure(const SkPoint* pts, SkSegType segType);
 
     SkScalar getTime(SkScalar targetLength);
     void getPosTanTime(SkScalar distance, SkPoint* pos, SkVector* tan, SkScalar* time);
     SkScalar getLength();
 
 private:
     const SkScalar kTolerance = 0.0001f;
     const int kNewtonIters = 5;
     const int kBisectIters = 5;
 
     SkSegType fSegType;
     SkPoint fPts[4];
     SkScalar fLength = -1.0f;
     ArcLengthIntegrator fIntegrator;
 
     // for debug purposes
     int fIters;
 };
 
 #endif  // SkCurveMeasure_DEFINED